Polishing of C# auth API

src/csharp/Grpc.Auth/GoogleAuthInterceptors.cs

src/csharp/Grpc.Auth/GoogleGrpcCredentials.cs

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Core.Tests/CallCredentialsTest.cs

src/csharp/Grpc.Core.Tests/ChannelCredentialsTest.cs

src/csharp/Grpc.Core/CallCredentials.cs

src/csharp/Grpc.Core/ChannelCredentials.cs

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

...csharp/Grpc.IntegrationTesting/MetadataCredentialsTest.cs